The integrity and security of communications links is an important aspect of communications and computer networks. The integrity of a communications link could be degraded by factors such as damage, degradation over time, and alteration of system components. These factors may be caused by environmental influences affecting the way interfaces behave (e.g., causing them to behave in unanticipated manners), by external forces such as stress, and by unauthorized circuit modifications after a system is fielded. Particularly problematic is damage or unauthorized modification caused by, for example, attempts to reverse engineer or capture intellectual property in commercial products, and attempts to capture financial data or otherwise eavesdrop on commercial communications. Conventional methods are often unable to detect obvious circuit modifications, and even less able to detect subtle ones.